In digital data cable modem systems, data is transmitted between a headend and a plurality of cable modems all of which are coupled to a hybrid fiber coaxial cable network. Because of imperfections in the system, such as taps which are not properly terminated, echoes from signals reflected from taps not properly terminated and ingress noise greatly affects the performance of the system, especially the upstream. To combat this noise, adaptive equalization has been used in the central unit receiver for combatting upstream noise and adaptive equalization in the cable modem receivers has been used to combat downstream noise.
Signal transmission channels have a property called dispersion which changes the shape of pulses which encode symbols being transmitted. Dispersion arises from the fact that every pulse is comprised of a plurality of Fourier components, each of which is a sinusoid of a different frequency and different amplitude and which, when added together, define the shape of the pulse. Dispersion and pulse shape changes arise from the fact that different frequency Fourier components propagate at different velocities. This phenomenon causes intersymbol interference or ISI between neighboring pulses, and ISI limits the number of discrete amplitude levels for symbol pulses which can be successfully detected. Equalization is a way of eliminating or reducing ISI.
If the exact characteristics of the channel are known, ISI can be eliminated or reduced substantially by using a pair of filters, one at the transmitter and one at the receiver to control the pulse shape distortion. The transmit filter is placed just before the modulator, and does pre-channel equalization. The receive filter is placed just after the demodulator and before the slicer, and does post-channel equalization. If the filter characteristics of these filters are set correctly, the transmit filter predistorts the pulse shapes so that the distortions in the channel do not cause ISI at the sample instants and the receive filter takes care of any remaining ISI noise before each received symbol is fed to the slicer for decision.
In practice however, the precise characteristics of the channel are rarely known in advance, and are time varying. In addition, there is always imprecision that arises in implementation of the filters. The net result is that there is always some residual distortion such that ISI will limit the data rate of the system. To compensate for this residual distortion, a process called equalization is used, and the filter which is used to do it is called an equalizer. Equalizers are adaptive usually to adjust to time varying needs for ISI reduction.
Adaptive equalizers are digital tapped delay line filters with impulse responses defined by the tap weights. These tap weights are called the filter coefficients. FIG. 1 is a block diagram of a typical prior art tapped delay line equalization digital filter. In synchronous equalizers, the taps are spaced along the delay line at the duration of the symbol. In some systems, only pre-channel equalization is used, but this requires a feedback channel if the pre-channel equalizer is adaptive. In most systems, post-channel adaptive equalization is used, and a training data sequence is sent before sending the payload data so that the post channel equalizer can adapt its coefficients for maximum ISI cancellation.
The adaptive equalization process involves setting tap weights, receiving training data and data symbols and processing it to determine whether slicer errors are occurring or will occur in reception of the data, then altering the tap weights and, sometimes, processing the training data again to determine if the number of errors was reduced. The process of adapting the tap weights to change the filter characteristics continues until the number of errors in reception is minimized, which is a state called convergence. Typically, adaptation is achieved by observing the error between the desired pulse shape and the actual pulse shape at the output of the equalizer filter, measured at the sampling instants, and then using this error to determine the direction in which the tap weights should be altered to approach an optimum set of values.
Equalization systems exist in at least two varieties: DFE and FFE. DFE stands for Decision Feedback Equalization and FFE stands for Feed Forward Equalization. Every channel through which symbols are transmitted has an impulse response which represents a transfer function and defines how the channel will affect a pulse propagating through it. In the sampled form, the impulse response of every channel has a term which represents the effect of precursors in the impulse response which occur before the main sample associated with the desired data symbol. The impulse response also has a term which is represents the effect of postcursors in the impulse response which happen after the main sample. FIG. 2 represents the precursor and postcursor parts of impulse response in sampled form. The idea of Decision Feedback Equalization is to use data decisions made on the basis of precursors of the channel impulse response to take care of the postcursors. For the idea to work however, the decisions have to be correct.
A DFE equalizer consists of a feedforward section, a feedback section, and a decision device, connected as shown in FIG. 3.
A consortium of cable system operators have formed Cable Labs as a body to develop standards for compatibility of the products of various headend and cable modem equipment manufacturers so that units from different manufacturers may be “plug-n-play”.
The first standard developed by Cable Labs and the members thereof was DOCSIS 1.0.
DOCSIS 1.X cable modems (hereafter any cable modem may be referred to as a CM) and DOCSIS 1.X Cable Modem Termination Systems (hereafter CMTS) only use FFE equalization filters. However, in DOCSIS 2.0 Cable Modem Termination Systems, both FFE and FBE equalization filters are used. This means that the FBE filter coefficients will feed back a signal to the summer which will reduce the post cursor efects on the data reaching the decision device, altering the decision error. This in turn will affect the adaptation of the FFE filter.
In DOCSIS 2.0, it is mandatory that all CMs use only FFE equalization filters.
Therefore, a need has arisen for a method to convert DOCSIS 2.0 DFE equalization coefficients to feed forward coefficients to match the DOCSIS requirement at the modem side.